The present invention relates to lighting fixtures and, more particularly to fluorescent lamp holder fittings which are adapted to screw into standard incandescent lamp sockets. The invention has particular application to lamp fixtures adapted to fit in three-way lamp sockets for producing three light output levels.
Three-way lamp sockets are typically used with special three-way incandescent lamps. The incandescent bulb is made with two filaments inside the bulb and three contact terminals on the base of the lamp. The socket has a three-way switch for operating the three-way lamp. In one switch position, the AC power supply is connected across a first pair of the lamp terminals for supplying power to a first one of the lamp filaments to give a first lamp output level. In a second switch position, the supply is connected across a second pair of the lamp terminals to supply power to a second filament to give a second light output level. In the third switch position, the supply is connected across both pairs of lamp terminals in parallel, to supply both of the filaments in parallel to give the third output level.
Energy saving compact fluorescent lamps are gradually replacing incandescent lamps, such fluorescent lamps being adapted to be screwed into the standard lamp socket. Efforts have also been made to provide three-way fluorescent lamps which can be used in three-way lamp sockets. For this purpose a number of types of dimmers has previously been used for varying the lamp current and/or voltage, but such prior arrangements have encountered a variety of problems, including cost, efficiency, reliability, power consumption and lamp start-up and running at low output levels.
Some manufacturers have tried to change the fluorescent lamp output by changing its operating frequency. However, such lamp units have required an extra input besides the line input, and have also required a separate switch. Such arrangements cannot be used with a standard three-way lamp socket of the type commonly used with incandescent three-way lamps.